This proposal concerns the function and regulation of ion transport systems found in the preimplantation mammalian embryo. The functions of ion transport will be investigated in two broad, related areas: intracellular pH regulation, and the transport of ions and fluid into the blastocoel cavity during its formation and maintenance. At the 2-cell stage of the mouse embryo, the only demonstrable intracellular pH regulatory system is the bicarbonate/chloride exchanger, which alleviates intracellular alkalosis; the mechanisms which alleviate acidosis and which are almost ubiquitous in other cells appear to be inactive in the embryo at this stage. It is likely, therefore, that the bicarbonate/chloride exchanger is important to the development of the embryo. The role of the bicarbonate/chloride exchanger and how its role changes over the course of development will be investigated. The capacity of preimplantation embryos to recover from alkalosis will be determined for each stage of development, and the mechanisms governing the recovery will be elucidated. The kinetics of the exchanger, and its molecular identity will be investigated. Finally, since chloride transport into the blastocoel is necessary, along with sodium, for the formation and maintenance of the blastocoel cavity and blastocoel fluid, the role of the bicarbonate/chloride exchanger and chloride transport mechanisms in general will be investigated to elucidate their roles in this process. It is expected that the information obtained from these studies will be useful in furthering the understanding of preimplantation embryo development in mammals, including in humans. The information will also have direct clinical relevance in the design and use of media for in vitro handling of embryos, as, for example, during in vitro fertilization for the treatment of infertility, and perhaps in identifying factors in the embryo which may be related to infertility.